I will take a look at your code because it's not easy to reproduce this problem in other applications. I think it is probably a redraw problem. Why NSToolbar can trigger such problem, I have no idea yet.

Sometimes, NSToolbar shows double toolbars here. I don't know why exactly that happens, but it seems to be reproducable for my application by opening and closing it several times without doing anything in the application in between. Here's a screenshot: